1. Field of the Invention
The present invention relates to a process for manufacturing a member having a sliding surface with a wear-resistant property, such as a cylinder liner for internal combustion engines.
2. Description of the Prior Art
Conventional internal combustion engines have cylinder blocks formed with cylinder bores in which pistons are disposed for slidable reciprocating movements. In each of the cylinder bores, there is a cylinder liner having an inner cylindrical surface which is adapted to be slidably engaged with the piston. Hithertofore, tremendous efforts have been made to improve the wear-resistant property of the sliding surface in the cylinder liner. It has been recognized that it is effective to improve the wear-resistant property by providing the sliding surface with pits for retaining lubricant oil. It has also been recognized that the quantity of the lubricant oil maintained in the pits is increased in response to an increase in the area ratio of the pits, that is, the ratio of the overall area of the pits to the area of the sliding surface. It should, however, be noted that a simple increase in the area ratio of the pits is not recommendable from the viewpoint of improving the wear-resistant property because the area of the plateaus is correspondingly decreased, causing an increase in the contact pressure in a unit area.
It is possible to increase the quantity of the lubricant oil retained in the pits without increasing the area ratio. For that purpose, the pits may be increased in depth but not in the area of the openings. However, the conventional reverse current process adopted for providing a porous plating layer is not suitable for increasing the depth of the pits, because an excessive reverse current electrolysis will produce rough surfaces in the plateaus. Photo-etching processes may be suitable for providing relatively deep pits in the sliding surface. The U.S. Pat. No. 2,968,555 issued on Jan. 17, 1961, to H. M. Bendler et al, proposes a process for treating metal surfaces in which the metal surfaces are formed with oil retaining pits through a photo-etching technique.
It should, however, be noted that the pits thus formed in the sliding surface simply retain the lubricant oil so that the lubricant oil retained in the pits is apt to be drawn out by a member which slides on the sliding surface. Since the quantity of the lubricant oil drawn from the pits increases as the sliding speed increases, there is a danger of shortage of lubricant oil depending on the sliding speed.
Improved lubrication will be accomplished by using a self-lubricating material. It has been a common practice to use a cast iron material containing graphites, such as flake graphites, in a member having a sliding surface. It should be noted, however, that in recent high power engines which are operated under a high output and a high speed condition, the sliding surface of the cylinder liner is subjected to a high temperature and a high contact pressure so that the graphites themselves cannot provide a satisfactory lubrication. In view of the problems, it has been proposed to spray a molten ceramic material on the sliding surface to form a wear-resistant layer. However, the ceramic layer thus formed does not have satisfactory durability because the ceramic layer cannot be attached to the substrate with a satisfactory bonding power.
It has also been proposed to provide the sliding surface with a composite plating layer having particles with a self-lubricant property distributed in the surface. However, a complicated process is required to form such a composite plating layer. A proposal has further been made to provide a sliding surface having particles of silicon carbides embedded in the surface through a mechanical process. However, this proposal is not advantageous either, because the process for distributing the silicon carbide particles is complicated and not practical.